The invention relates to a method of protecting against corrosion a monocrystalline superalloy containing at least one refractory metal, in which is formed on the surface of the superalloy a coating containing aluminium.
With the purpose of optimising temperature-resistance and oxidation-resistance of parts of turboengines composed of superalloys, these are covered with a protective coating containing aluminium in order to form at the surface of the part covered a protective aluminium oxide. This coating may be formed by conventional aluminisation treatment, which may be either high-activity or low-activity, e.g. low-activity vapour-phase aluminisation. However, the aluminium present in this coating migrates as much towards the surface of the part where it renews the oxide layer as towards the superalloy substrate, of which it impairs the features of use (the aluminium is then the chemical motor of this impairment) and in so far reduces the amount available to renew the oxide layer.
In order to improve the mechanical properties of turboengine parts composed of nickel-based superalloys, compositions have been developed which are rich in refractory elements and at the limit of stability, the limit of solubility of these elements in they phase being affected.
After the development of a coating on this type of superalloy with a γ/γ′ structure, there appear in the layer known as the “diffusion layer” located between the coating and the substrate microstructural weaknesses leading to the formation of phases said to be TCP (Topologically Close Packed) and progressive secondary reaction zones (SRZ). These form in the part of the diffusion layer closest to the substrate, known as the interdiffusion zone.
The affect of the secondary reaction zones on the mechanical properties is still poorly understood. However, the mere fact that a secondary reaction zone, whose thickness may vary from 20 to 100 μm according to the quantity of available aluminium, forms under the diffusion zone, whose thickness is typically of the order of 20 μm, reduces by that amount the thickness of healthy alloy. This may be particularly harmful in the case where a thin-walled component is being used, such as cooled blades. Therefore, numerous work has been carried out to identify the causes of secondary reaction zones and to reduce them, if not eliminate them.
The nature of the substrate and its chemical composition (in particular monocrystalline alloy rich in rhenium and low in cobalt) seem to play a part in determining the appearance of secondary reaction zones.
In the conditions of use of the parts, the secondary reaction zones increase towards the interior thereof, further reducing their mechanical strength over the course of time.
Local stresses also favour the appearance of secondary reaction zones. These local stresses are due to operations prior to any coating (sand-blasting in particular) (mechanical motor).
The analysis of a secondary reaction zone shows that it is formed of filaments γ in a matrix γ′. An incoherent grain boundary separates the secondary reaction zones from the γ/γ′ structure of the superalloy.
Some authors have sought to overcome the mechanical motor by reducing the stresses by recrystallising a thin surface zone of the superalloy before proceeding to the stages of forming the coating (Rebecca A MacKay, Ivan E Locci, Anita Garg, Frank J Ritzert, Techniques Optimized for Reducing Instabilities in Advanced Nickel-base Superalloys for Turbine blades, RT2001 NASA Technology report, NASA TM 2002-211333; W H Murphy, W S Walston, Method for making a coated Ni base superalloy article of improved microstructural stability, U.S. Pat. No. 5,695,821).
Others specify changes in composition (U.S. Pat. No. 5,695,821; K S O'Hara, W S Walston, E W Ross, R Darolia, Nickel base superalloy and article, U.S. Pat. No. 5,482,789) or carbiding treatments (J Fernihough, Process for strengthening the grain boundaries of a component made from a Ni based superalloy, U.S. Pat. No. 6,471,790; J Schaeffer, A K Bartz, P J Fink, Method for preventing recrystallisation after cold working superalloy article, U.S. Pat. No. 5,598,968), or of nitriding (K S O'Hara, W S Walston, J C Schaeffer, Substrate stabilisation of superalloy protected by an aluminium-rich coating, U.S. Pat. No. 6,447,932). These latter specifications have the aim of creating carbides or nitrides which would pin down the secondary reaction zones and would inhibit their progression.
Kelly et al. (T J Kelly, P K Wright III, Article having a superalloy protective coating and its fabrication, U.S. Pat. No. 6,641,929) propose to deposit a metal layer by cathode sputtering before the protective operation. This layer is no other than a second superalloy, the γ/γ′ alloy interdiffusion not leading to the formation of secondary reaction zones.
Finally, R G Wing teaches (Method of aluminizing a superalloy, U.S. Pat. No. 6,080,246) that it is possible to stabilise the composition of the surface of superalloys heavily enriched in refractory elements (Re and/or Ru) by the diffusion of a deposit of cobalt or a deposit of chromium, the latter being preferably deposited by thermochemical means (chromising). However, in the case where cobalt is used, although this technique makes it possible to do away with secondary reaction zones, it leads to the formation of a protective coating heavily enriched in this element. In an article by Warnes et al. (Cyclic oxidation of diffusion aluminide coatings on cobalt base super alloys, Bruce M Warnes, Nick S DuShane, Jack E Cockerill, Surface and Coatings Technology 148 (2001) 163-170), it is stated in conclusion that the coatings obtained by diffusion on cobalt-based alloys (the aluminides of cobalt) are probably insufficient to protect the turbines in operation. This technique therefore makes it possible to retain the microstructure of the alloy but to the detriment of its resistance to oxidation.